The synthesis of Pd-Ag composite membranes for H2 separation using electroless plating method

Bhandari, Rajkumar ms

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The synthesis of Pd-Ag composite membranes for H2 separation using electroless plating method

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One of the key elements to the success of Pd-Ag membrane based reactor for the H2 production is the synthesis of thin and highly selective membranes using the electroless plating method. This work describes the effect of electroless plating conditions on the obtained Pd and Ag deposits properties (morphology, compactness, phase structure, compositional homogeneity and adhesion) important from synthesis of thin and H2 selective membrane viewpoint. Both sequential and co-deposition deposition methods were investigated. The conventional Pd and Ag plating conditions (NH3+EDTA based bath) produced dendritic and non-uniform sequential (multi layer) deposits, not suitable for synthesizing the thin and H2 selective Pd-Ag membranes. Ag under the conventional plating conditions deposited at high overpotential resulting in the dendritic and non-uniform sequential deposits. The modified Ag plating conditions eliminated Ag deposition at high overpotential and the sequential deposits obtained were non-dendritic and uniform. Thin (< 10 Âµm thick) and H2 selective Pd-Ag membranes were successfully synthesized using the modified Ag plating conditions. The membranes were then successfully annealed at 550 oC. After the annealing step, the membranes showed activation energy for the H2 permeation (4.3â€“11.5 kJ/mole) lower than that of the pure Pd membrane (12-16.4 kJ/mole) meaning that the Pd-Ag membranes were more effective for the H2 separation at lower temperatures than the pure Pd membrane. A Pd-Ag (20 wt%) membrane showed H2 permeance higher by a factor of 2.47 at 250 oC than the pure Pd foil. The Pd-Ag membranes also showed decline in the H2/He selectivity on exposure to the annealing and H2 permeation (300-500 oC) study conditions. The Pd-Ag co-deposits obtained (using NH3+EDTA bath) were dendritic, inhomogeneous with poor substrate adhesion, therefore not suitable for the membrane synthesis. The co-deposits were bi-metallic and required the annealing step to form the Pd-Ag alloy. There existed a large difference in the deposition potentials (600 to 650 mV) of Pd and Ag. The Ag deposition was severely controlled by its mass transfer in the solution resulting in the dendritic and inhomogeneous deposits. Among the different complexing agents investigated, KCl showed the least difference between the Pd and Ag deposition potentials. The co-deposits obtained using the KCl bath were non-dendritic, homogeneous and were Pd-Ag alloy therefore required no annealing step. Finally, the multi step plating, annealing and polishing approach was used to avoid the decline in the selectivity of the sequentially prepared Pd-Ag membranes. The membranes prepared by the plating, annealing and polishing approach showed very high selectivity (H2/He) and no decline in the selectivity was observed between 300-450 oC for the total exposure time > 550 h (> 200 h at 450 oC).

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